In a production process in fields where it is necessary to work in a clean space, such as the semiconductor or electronic device industry, the work is carried out in a clean room. Therein, as a means to manage the clean room, a light scattering type particle counter is known, this counts the number of particles floating in the air.
The light scattering type particle counter is provided with a sensor block which detects a scattered light from a particle when a gas sample is irradiated with light from a light source. In this sensor block, an inlet nozzle and an outlet nozzle are disposed at a predetermined interval, an opened space exists between the inlet nozzle and the outlet nozzle, a linear flow path is formed in this space, and a sample gas flows from the inlet nozzle into the outlet nozzle through the flow path. In this type, a detection area is formed by irradiating the flow path with the light from a light source.
A slight turbulence may be generated in the flow path including the detection area, the particles which do not flow into the outlet nozzle are left in the sensor block, and drift in the sensor block. Such particles are called floating particles and the floating particles cause a false counting (miscalculation) in the particle detection.
To improve the false counting, Japanese Laid Open Patent No. Hei 11-258145 discloses a method to decrease false counting caused by floating particles (vagrancy particle), that is, to detect whether a wave pattern (a voltage wave pattern) of a particle is less than a baseline within a predetermined time, using the continuance time of the wave pattern of the atmospheric floating particle as being long.
Besides the false counting due to floating particles, a numerical value may be indicated even if a particle does not exist in a sample. This may be caused by a photoelectric conversion element (photodiode) reacting to an electromagnetic wave, a cosmic ray, whose radiation invades from the outside (it writes below down with radiations) or a change of the intensity of irradiation light.
To counteract such a false counting, Japanese Laid Open Patent No. 2006-177687 discloses a particle counter which subtracts a generation frequency of the false counting determined (calculated) beforehand from a counting after the start of measurement.
However, regarding the method disclosed in Japanese Laid Open Patent No. Hei 11-258145, if a duplicated wave pattern of a plurality of normal particles (subject of measurement) cannot be distinguished from the wave pattern of the floating particle, then it cannot count the number of the normal particles.
And, if it cannot distinguish the wave pattern due to radiation and a change of intensity of irradiation light, it may miscount by mistake.
Also, regarding the particle counter disclosed in Japanese Laid Open Patent No. 2006-177687, it does not take into consideration the floating particles, and subtracts the value that is predicted from generation frequency from measurements, it may be different from the actual value.